1. Field of the Invention
The present invention relates to an exhaust gas purifying method of an internal combustion engine using a NOx absorber catalyst.
2. Background Art
Hitherto in exhaust gas purifying system using a NOx absorber catalyst, NOx exhausted in a lean operation state of an internal combustion engine is absorbed by the NOx absorber catalyst, the operation state of the internal combustion engine is changed to a rich operation state so that NOx absorbed by the NOx absorber catalyst is released and reduced at the same time at every predetermined period (as is disclosed in, for example, the Japanese Patent No. 2600492).
It is generally known that a catalyst does not sufficiently function below a predetermined temperature (activation temperature). The NOx absorber catalyst does not sufficiently function below the activation temperature, either. Therefore, air-fuel ratio control of the conventional exhaust gas purifying method using the NOx absorber catalyst also includes any process such as inhibiting lean operation in the case where it is judged that the NOx absorber catalyst is below a predetermined temperature.
However, when studying in detail, it is found that the temperature characteristics of absorbing and releasing NOx of the NOx absorber catalyst is shown as FIG. 4. In FIG. 4(a), C-Nox is an absorbing capacity of the NOx absorber catalyst. In FIG. 4(b), Vo is a velocity of absorbing NOx of the NOx absorber catalyst. In FIG. 4(c), Ve is a velocity of releasing NOx of the NOx absorber catalyst. And in FIG. 4, T is a temperature of the NOx absorber catalyst.
In FIG. 4, it is found that as long as the NOx absorber catalyst is in a temperture range from T0 to T1, it is possible to absorb NOx but NOx is not released (releasing speed is 0), and in a temperature range from T1 to T2, NOx is sufficiently absorbed but is not sufficiently released (releasing speed is low). This means that sufficient effect is not obtained without giving due consideration to the temperature at the time of making the air-fuel ratio rich to reduce NOx.
Several inventions have been proposed giving consideration to temperature of the NOx absorber catalyst. For example, temperature of the catalyst has been considered in estimating the amount of NOx actually absorbed and estimating the amount of NOx that can be absorbed. Temperature of the NOx absorber catalyst is also considered when NOx is released in order to reactivate the ability for absorbing NOx once deteriorated due to sulfur oxide (Sox). However, none of the conventional inventions has given consideration to the temperature of the NOx absorber catalyst at the time when the air-fuel ratio is changed to the rich side in order to release and reduce the absorbed Nox. In particular, no consideration has been given to the fact that it is impossible to sufficiently release the absorbed NOx when the NOx absorber catalyst is at a low temperature.
Therefore. A problem exists in the conventional systems that the air-fuel ratio is changed to the rich side despite that the temperature of the NOx absorber catalyst is in a range where the NOx absorber catalyst cannot sufficiently release NOx, which eventually results in increase in toxic components (HC and CO) of the exhaust gas, otherwise inhibiting lean operation in this temperature range thereby reducing the advantage of less fuel consumption.
The present invention was made to solve the above-discussed problems, and intends to prevent exhaust gas from increase in toxic components and secure a lean operation time and favorable fuel consumption by inhibiting changing air-fuel ratio to the rich side otherwise by temporarily changing the air-fuel ratio nearly to a stoichiometric air-fuel ratio to raise temperature of the NOx absorber catalyst and then gradually making the air-fuel ratio rich, in the case where the temperature of a NOx absober catalyst is lower than a predetermined temperature.
A method of purifying exhaust gas of an internal combustion engine according to the invention utilizes a NOx absorber catalyst. This NOx gas occlusion catalyst absorbs NOx when oxygen concentration of exhaust gas flowing in is excessively increased, while releases NOx and reduces the NOx at the same time when the oxygen concentration of the exhaust gas is lowered. For the purpose of releasing NOx absorbed by the mentioned NOx absorber catalyst and reducing the NOx at the same time, an air-fuel ratio of the exhaust gas flowing into the mentioned NOx absorber catalyst is compulsorily changed from the lean side to the rich side.
More specifically, first, a temperature range where the mentioned NOx absorber catalyst can absorb NOx but does not release NOx is assumed as a range from a predetermined temperature T0 to a first predetermined temperature T1. Then a temperature range where the mentioned NOx absorber catalyst can sufficiently absorb NOx but does not sufficiently release NOx is assumed as a range from the first predetermined temperature T1 to a second predetermined temperature T2.
If the temperature of the mentioned NOx absorber catalyst is lower than the second predetermined temperature T2, the air-fuel ratio at the time of temporarily changing the air-fuel ratio of the exhaust gas to the rich side to release and reduce NOx is set depending on the temperature of the NOx absorber catalyst.
As a result, in the mentioned method of purifying exhaust gas of an internal combustion engine according the invention, if it is judged that the temperature of the NOx absorber catalyst is lower than the second predetermined value T2, the air-fuel ratio at the time of changing the air-fuel ratio to the rich side to release and reduce the absorbed NOx is set depending on the temperature of the NOx absorber catalyst. It is therefore possible to maintain desirable fuel consumption without increasng the toxic components of the exhaust gas and simplify the air-fuel ratio control.
Another method of purifying exhaust gas of an internal combustion engine according the invention utilizes means for controlling an air-fuel ratio of the internal combustion engine by changing the air-fuel ratio to a lean air-fuel ratio, a stoichiometric air-fuel ratio or a rich air-fuel ratio according to operating conditions of the internal combustion engine and a NOx absorber catalyst for absorbing NOx when an oxygen concentration of exhaust gas flowing in is excessively increased and releasing NOx and reducing the NOx at the same time when the oxygen concentration of the exhaust gas is lowered. At the time of changing the air-fuel ratio from the lean air-fuel ratio to the stoichiometric air-fuel ratio according to the operating conditions of the internal combustion engine, the air-fuel ratio is changed to the stoichiometric air-fuel ratio after the air-fuel ratio has been compulsorily changed to the rich air-fuel ratio. Thus NOx absorbed by the NOx absorber catalyst during the lean operation is released and is reduced at the same time.
In the mentioned air-fuel ratio at the time of temporarily changing the air-fuel ratio of the exhaust gas to the rich side, the air-fuel ratio is set depending on temperature of the mentioned NOx absorber catalyst.
As a result, in the mentioned method of purifying exhaust gas of an internal combustion engine of the invention, at the time of changing from operation with lean air-fuel ratio to operation with stoichiometric air-fuel ratio due to any variation in operating conditions of the vehicle, the air-fuel ratio is temporarily changed compulsorily to the rich air-fuel ratio to release and reduce NOx absorbed during the lean air-fuel ratio operation. In such changing operation, the rich air-fuel ratio is set depending on catalyst temperature, and as a result it is possible to favorably reduce the exhaust gas and simplify the arrangement of air-fuel ratio control.